The invention relates to a process for generating a fuel gas for a gas turbine by separating off sulphur components from a feed gas comprising hydrogen, carbon monoxide, carbon dioxide and also carbonyl sulphide and/or hydrogen sulphide, which feed gas is in this case scrubbed in a physical gas scrubber with a methanol scrubbing medium at a pressure between 30 and 80 bar(a) in order to obtain a first sulphur-free gas mixture and also a methanol scrubbing medium loaded with sulphur components and carbon dioxide, which methanol scrubbing medium is then regenerated, wherein a carbon dioxide-rich gas phase that contains sulphur component and is formed by pressure reduction is treated in a further gas scrubber with a methanol scrubbing medium in order to back-wash sulphur components and to generate a second sulphur-free gas mixture.
In addition, the invention relates to a device for carrying out the process according to the invention.
The methanol scrubbing medium used in the physical gas scrubber is technically pure methanol or a mixture of substances predominantly consisting of methanol, which mixture also contains, in particular to improve the scrubbing properties, other substances.
Sulphur-free or desulphurized in the context of the present disclosure is a substance or mixture of substances the sulphur component content of which falls below a preset limiting value. Complete sulphur freedom, which in any case cannot be achieved without considerable expenditure, is not necessary.
Physical gas scrubbers are used to separate off defined components from gas mixtures. They exploit the property of liquids to absorb gaseous substances and keep them physically bound in solution. How well a gas is absorbed by a liquid is expressed by the solubility coefficient which is greater the better the gas dissolves in the liquid. The solubility coefficient generally increases with falling temperature and, according to Henry's absorption law, with increasing pressure.
If a gas component i is to be extracted from a gas mixture by physical gas scrubbing, for this purpose a minimum quantity Wmin of the liquid used as scrubbing medium is necessary, which quantity may be calculated readily with the following formula:Wmin=V/(p·λi)
In the formula, V denotes the total amount of the gas mixture, p the pressure prevailing in the gas mixture and λ the solubility coefficient of the gas component to be extracted with respect to the scrubbing medium used. Under the assumption that the solubility coefficients of the components of a gas mixture differ sufficiently, by an appropriate adaptation of the amount of scrubbing medium it is possible, in a scrubbing step to separate off the gas component having the greatest solubility coefficient substantially independently of the remaining gas components, i.e. to remove it selectively. The scrubbing medium in this case is loaded with the components that are separated off.
Subsequently to the gas scrubbing, the loaded scrubbing medium is regenerated by removing the dissolved gas components. The regenerated scrubbing medium is usually reused for the gas scrubbing, whereas the gas components that are removed are either disposed of or fed to an economic utilization.
In particular for purifying synthesis gases, gas mixtures which are generated on an industrial scale in gasification plants from coal and/or hydrocarbon-containing feeds, for example by reforming with steam or by partial oxidation, and which generally, in addition to the desired substances hydrogen, carbon monoxide and optionally carbon dioxide, also contain some unwanted constituents such as the sulphur components hydrogen sulphide and carbonyl sulphide, physical gas scrubbers are used. These processes are considered useful, since synthesis gases are now usually generated at high pressure, and the efficacy of physical gas scrubbing increases, to a first approximation, linearly with the operating pressure. Methanol scrubbing is of particular importance for purifying synthesis gases. It exploits the fact that the solubility coefficients of hydrogen sulphide, carbonyl sulphide and carbon dioxide with respect to the methanol scrubbing medium used differ greatly from those of hydrogen and carbon monoxide. Since with falling temperature these differences increase, and thereby the hydrogen losses and carbon monoxide losses by co-absorption decrease, the methanol scrubbing medium is usually introduced into a scrubbing column far below 0° C. and brought into intense contact with the synthesis gas that is to be purified.
If the gas mixture containing hydrogen and carbon monoxide that was generated from the synthesis gas by gas scrubbing is to be fed, for example, to a gas turbine as fuel, it is necessary to attempt to remove the sulphur components as completely as possible, since these firstly would lead to damage of the gas turbine and secondly applicable emission standards in the exhaust gas of the gas turbine must be complied with. At the same time, it is desirable to leave the carbon dioxide virtually completely in the gas mixture, since it is required as a temperature moderator and working medium in the gas turbine. It is prior art to subject the synthesis gas to a single-stage methanol scrubbing in which the minimum amount of unloaded methanol scrubbing medium is used for separating off the sulphur components. In this case, a substantially sulphur-free gas mixture is formed, with, however, markedly decreased carbon dioxide content, since, in addition to the sulphur components, carbon dioxide is also absorbed to a considerable extent by the methanol scrubbing medium.
For the regeneration, the methanol scrubbing medium loaded with sulphur components and carbon dioxide is first expanded to a first pressure in order to desorb co-absorbed hydrogen and co-absorbed carbon monoxide and to increase the yield to recirculate them via a compressor into the synthesis gas. Subsequently, the methanol scrubbing medium is subjected to a hot regeneration, in which a substantially sulphur-free methanol scrubbing medium and also a gas phase comprising sulphur components and carbon dioxide are formed. Since the gas phase is customarily fed to an appliance for sulphur recovery, in which carbon dioxide is only unwanted ballast, it is expedient if it has a sulphur content as high as possible.
For this purpose, a cold regeneration can be connected upstream of the hot regeneration, in which cold regeneration the methanol scrubbing medium which is expanded to the first pressure and cooled in the course of this is further expanded into what is termed an enrichment column to a second pressure situated only slightly above atmospheric pressure, in order to liberate the largest possible fraction of the carbon dioxide present into the gas phase. Sulphur components that are also liberated are backwashed with regenerated methanol scrubbing medium from the gas phase conducted upwards in the enrichment column, in such a manner that a methanol fraction depleted in carbon dioxide content is obtained for passing on to the hot regeneration, and also a carbon dioxide-rich gas phase is obtained which is disposed of as tail gas.
Owing to the low pressure in the enrichment column, the requirement for regenerated methanol scrubbing medium for the backwash of the sulphur components is high. Since, in addition, the low temperatures make liberation of the carbon dioxide more difficult, it is necessary to support the cold regeneration by a stripping, for which purpose, usually nitrogen is introduced as stripping gas into the enrichment column.
According to the prior art, carbon dioxide that is scrubbed out of the synthesis gas is lost with the tail gas in such a manner that the sulphur-free gas mixture obtained by the gas scrubbing, owing to the carbon dioxide content thereof, is ultimately only of limited suitability for use in a gas turbine.